I. Technical Field
The present invention relates, for example, to an undercut-free constant velocity universal joint used in a front-wheel drive shaft of an automobile.
II. Description of the Related Art
The constant velocity universal joint is used in a power transmission system of an automobile or various industrial machines, and connects a rotating shaft on a driving side to a rotating shaft on a driven side so as to transmit torque at constant angular velocity. The constant velocity universal joint includes a fixed type one and a plunging type one. While the fixed type joint allows only angular displacement, the plunging type joint allows both the angular displacement and axial displacement.
The fixed type constant velocity universal joint used in a wheel side of the front-wheel drive shaft of an automobile is required to allow a high operating angle because wheels move right and left in connection with operation of a steering. Therefore, this constant velocity universal joint is required to have static strength strong enough to endure even if a large driving force (torque) is applied at the high operating angle, and a fatigue strength characteristic sufficient for preventing abrupt fracture even if the automobile is driven over a long distance within the warranty period.
The fixed type constant velocity universal joint includes a Rzeppa joint (hereinafter, referred to as BJ) and an undercut-free joint (hereinafter, referred to as UJ). Each of the BJ and UJ includes an outer joint member having a cup portion opening at one end, an inner joint member positioned in the interior of the cup portion, balls interposed between the outer joint member and an inner joint member to transmit torque, and a cage for retaining the balls therein. Further, the cup portion of the outer joint member has track grooves which are formed in an inner surface thereof and along which the balls roll, and the inner joint member has track grooves which are formed in an outer surface thereof and along which the balls roll.
In the BJ, the track grooves of the cup portion of the outer joint member and the track grooves of the inner joint member have axial sections of a circular-arc shape. In contrast, in the UJ, the circular-arc-shaped axial sections in the BJ of the track grooves of the cup portion of the outer joint member and the track grooves of the inner joint member are partially changed to a straight shape. As illustrated in FIG. 6A, an axial section of track grooves 23 of a cup portion 22 of an outer joint member 21 in the UJ has a straight shape (hereinafter, referred to as “straight portion”) on an opening side.
In the UJ, a contact point between each ball 24 and each track groove 23 is positioned radially outward at an opening end portion of the track groove 23 of the cup portion 22 of the outer joint member 21 in comparison with the BJ. Therefore, a shaft interferes with the cup portion 22 of the outer joint member 21 on a further high operating angle side in comparison with the BJ, and hence it is possible to allow formation of the high operating angle. Meanwhile, in order to ensure that the UJ allows the high operating angle, it is necessary to make permissible the load higher than that of the BJ. Therefore, the UJ generally has the cup portion 22 which has a larger outer diameter than that of the BJ so as to ensure strength. However, there is a need for reduction in size of the cup portion 22 of the UJ. Therefore, for the reduction in size, it is necessary to further increase permissible stress of the cup portion 22.
In order to increase strength of the cup portion 22 of the outer joint member 21, it is necessary to increase strength of portions which are prone to be fractured of the cup portion 22. The portions which are prone to be fractured of the cup portion 22 are vicinities of the opening end portions of the track grooves 23. As one method for increasing strength of the opening end portions of the track grooves 23, hardening by quenching is exemplified. The hardening by quenching provides an effect of suppressing plastic deformation of track opening portions. As quenching for suppressing deformation of the opening end portions of the track grooves 23, various methods have already been devised.
For example, the invention described in JP 57-67127 A discloses the following method. For the purpose of providing compressive residual stress so as to increase fatigue strength of the opening end portion of the outer joint member 21, an excess thickness is provided in the opening end portion of the outer joint member 21 in advance, and the excess thickness of the opening end portion is eliminated after quenching.
Further, the invention described in JP 4-194418 A discloses the following method. The opening end portions of the track grooves 23 of the outer joint member 21 which are subjected to induction quenching are further subjected to shot peening so as to be provided with compressive residual stress, to thereby increase the fatigue strength of the opening end portions of the track grooves 23 of the outer joint member 21.
Further, in the invention described in JP 56-65924 A, a heat-absorbing body is brought into contact with the opening end portions of the track grooves 23 of the outer joint member 21 so as to decrease depth of hardened layers in the opening end portions of the track grooves 23. This is because, when the depth of the hardened layers formed by induction quenching of the opening end portions of the track grooves 23 of the outer joint member 21 is increased, unhardened layers are reduced, to rather decrease the fatigue strength.